Conditional command line arguments in Python using argparse

Question

I'd like to have a program that takes a --action= flag, where the valid choices are dump and upload, with upload being the default. If (and only if) dump is selected, I'd like there to also be a --dump-format= option. Is there a way to express this using argparse, or do I need to just accept all the arguments and do the logic myself.

Answer 1

The argparse module offers a way to do this without implementing your own requiredness checks. The example below uses "subparsers" or "sub commands". I've implemented a subparser for "dump" and one for "format".

import argparse  parser = argparse.ArgumentParser() parser.add_argument('file', help='The file you want to act on.') subparsers = parser.add_subparsers(dest='subcommand') subparsers.required = True  # required since 3.7  #  subparser for dump parser_dump = subparsers.add_parser('dump') # add a required argument parser_dump.add_argument(     'format',     choices=['csv', 'json'],     help='Dump the file in this format.')  #  subparser for upload parser_upload = subparsers.add_parser('upload') # add a required argument parser_upload.add_argument(     'server',     choices=['amazon', 'imgur'],     help='Upload the file to this service.')  args = parser.parse_args() print args if args.subcommand == 'dump':     print 'I will now dump "%s" in the %s format' % (args.file, args.format) if args.subcommand == 'upload':     print 'I will now upload "%s" to %s' % (args.file, args.server) 

That looks like this on the command line:

$ python ap.py  usage: ap.py [-h] file {upload,dump} ... ap.py: error: too few arguments $ python ap.py tmp.txt  usage: ap.py [-h] file {upload,dump} ... ap.py: error: too few arguments $ python ap.py tmp.txt upload usage: ap.py file upload [-h] {amazon,imgur} ap.py file upload: error: too few arguments $ python ap.py tmp.txt upload amazo usage: ap.py file upload [-h] {amazon,imgur} ap.py file upload: error: argument server: invalid choice: 'amazo' (choose from 'amazon', 'imgur') $ python ap.py tmp.txt upload amazon Namespace(file='tmp.txt', server='amazon', subcommand='upload') I will now upload "tmp.txt" to amazon $ python ap.py tmp.txt upload imgur Namespace(file='tmp.txt', server='imgur', subcommand='upload') I will now upload "tmp.txt" to imgur $ python ap.py tmp.txt dump usage: ap.py file dump [-h] {csv,json} ap.py file dump: error: too few arguments $ python ap.py tmp.txt dump csv Namespace(file='tmp.txt', format='csv', subcommand='dump') I will now dump "tmp.txt" in the csv format $ python ap.py tmp.txt dump json Namespace(file='tmp.txt', format='json', subcommand='dump') I will now dump "tmp.txt" in the json format 

More info: http://docs.python.org/dev/library/argparse.html#argparse.ArgumentParser.add_subparsers